Heather Schrum

Subseafloor sedimentary life in the South Pacific Gyre

The low-productivity South Pacific Gyre (SPG) is Earths largest oceanic province. Its sediment accumulates extraordinarily slowly (0.1–1 m per million years). This sediment contains a living community that is characterized by very low biomass and very low metabolic activity. At every depth in cored SPG sediment, mean cell abundances are 3 to 4 orders of magnitude lower than at the same depths in all previously explored subseafloor communities. The net rate of respiration by the subseafloor sedimentary community at each SPG site is 1 to 3 orders of magnitude lower than the rates at previously explored sites. Because of the low respiration rates and the thinness of the sediment, interstitial waters are oxic throughout the sediment column in most of this region. Consequently, the sedimentary community of the SPG is predominantly aerobic, unlike previously explored subseafloor communities. Generation of H2 by radiolysis of water is a significant electron-donor source for this community. The per-cell respiration rates of this community are about 2 orders of magnitude higher (in oxidation/reduction equivalents) than in previously explored anaerobic subseafloor communities. Respiration rates and cell concentrations in subseafloor sediment throughout almost half of the world ocean may approach those in SPG sediment.

Sulfate-reducing ammonium oxidation: A thermodynamically feasible metabolic pathway in subseafloor sediment

Biogeochemical fluxes and Gibbs energies in sedimentary pore-waters point to the existence of sulfate-reducing ammonium oxidation. This process has not been previously inferred in natural environments. Porewater profiles in the Bay of Bengal (Indian Ocean) demonstrate that significant ammonium disappears at the ammonium-sulfate interface. Loss of ammonium at this horizon greatly exceeds possible nitrogen demand by biomass production. In situ Gibbs energies of reaction (ΔG) in Bay of Bengal and Greenwich Bay (Rhode Island) sediments indicate that sulfate-reducing ammonium oxidation is energy yielding. Relatively small and constant but consistently negative ΔG values for this reaction in both locations match the thermodynamic signature of anaerobic microbial respiration. The ΔG results and the substantial ammonium loss suggest that sulfate-reducing ammonium oxidation occurs in Bay of Bengal sediment. The Greenwich Bay ΔG results suggest that the process may also occur in anoxic sediment where the ammonium concentration profile shows no net loss of ammonium.